Controlling the transport and structural properties of oxide thin films through various parameters (such as temperature, strain, and electric field) makes them useful for technological applications including sensor, memory and logic devices. Recently, a mechanism was demonstrated for controlling the properties of a class of oxide materials, namely, gating them with ionic liquids. The voltage gating of an ionic liquid (IL) at the surface of an oxide film can create an electric field large enough that oxygen migrates from within the interior of the film to its surface, as illustrated in FIG. 1. This process is reversible and can be used with a large class of oxides having channels through which oxygen ions migrate. Such an ion transport channel is a collection of lattice sites along which oxygen ion diffusion occurs, as opposed to diffusion through one or more mesoscopic pores.
Of particular interest are the oxides VO2 and WO3, which can be reversibly gated for thicknesses at least as large as ˜120 nm. One consequence of the IL gating is the change in conductivity of the oxide films. In particular, for VO2, which displays a metal to insulator transition (MIT) near room temperature, one observes a suppression of this MIT even at low temperatures. The observed conductivity increase of the insulating state upon IL gating (application of positive gate voltage) is roughly three orders of magnitude and is non-volatile (see Jeong et al., “Suppression of metal-insulator transition in VO2 by electric-field induced oxygen vacancy formation”, Science, vol. 339, pp. 1402-1405, 2013). The material retains its conducting properties even when the bias voltage is reduced to zero and, further, even after the removal of the ionic liquid. The original high resistance state of the pristine material can be reached upon application of a reverse gate voltage (negative gate voltage). Upon IL gating (application of a positive gate voltage) in the case of WO3, which is a band insulator, one observes an increase in conductance of almost six orders of magnitude (see Altendorf et al., “Facet-independent electric-field-induced volume metallization of tungsten trioxide films”, Advanced Materials, 2016). For both of these oxides, the original insulating state is reached during reverse gating by migration of oxygen from the surface of the film and/or the ionic liquid to its interior. This gate-induced migration of oxygen in and out of the oxide film demonstrates the utility of the oxide film for the transport of oxygen across the film thickness. FIG. 2 shows an IL gating effect for a 10 nm VO2 thin film. The pristine film shows an MIT characteristic of VO2, which is suppressed by IL gating due to the creation of oxygen vacancies.
One observes a clear correlation between the increased conductivity of the film and the removal of oxygen from the film, while the converse is also true. In particular, the film conductivity depends on the oxygen present in the environment during the IL gating process. The presence of a sufficiently high concentration of oxygen can completely suppress any gate-induced conductivity increase. Other gases including nitrogen or argon have no significant effect on the IL gating process (see Li et al., “Suppression of ionic liquid gate-induced metallization of SrTiO3(001) by oxygen”, ACS Nano, vol. 13, pp. 4675-4678, 2013). This is evidence of an extremely high specificity of the IL gating process to oxygen.
High purity oxygen finds wide-ranging applications in many fields such as medicine, manufacturing, energy storage and transportation. Existing techniques for the separation of oxygen from air include rectification, selective adsorption in the pores of zeolite-based materials and/or the use of porous gas separating membranes. The primary challenge in separating oxygen is distinguishing it from nitrogen, which is the primary constituent of air. Molecular nitrogen is similar in size to molecular oxygen and therefore is difficult to separate from oxygen using processes that depend on adsorption or gas permeation. Furthermore, the two gases have very similar boiling points, thereby limiting the separation efficiency in air “rectification” techniques. In order for these processes to be effective, they require either low temperatures (˜−190° C.) or high pressures or both. Although they are attractive for mass production of oxygen, in general these techniques produce low purity oxygen after single stage purification. Multiple stages of purification are required to obtain higher purity. This is a serious limitation when high purity oxygen is required on a small scale. For example, on-demand production of high purity oxygen is essential for a metal-air battery system. Although mixed ionic and electronically conducting ceramic membranes have also been proposed for gas separation, these approaches still require high temperatures (˜800-900° C.) and high pressures to enable efficient gas separation. In short, state of the art techniques require extensive gas handling apparatuses, making them bulky and unsuitable for portable applications requiring high purity oxygen.
These stringent operating conditions require gas separation membranes possessing high thermal stability, chemical inertness and mechanical integrity. Furthermore, gas separation membranes should be constructed such that they can withstand high pressure-differentials and/or high temperatures.